This invention relates to a method of improving the quality of non-destructive examination (NDE) images, and more particularly to incorporating a priori information into incomplete data NDE images.
Good image quality and substantial information about a part are provided when the images are based on a complete data set, for instance that there are sufficient spatial an angular samples to create an image consistent with part and flaw geometry. However, complete data for image reconstruction is not always available. Consider X-ray CT (Computerized Tomography) as an example. In certain cases it may be impossible to manipulate a part in such a way that data can be taken at all the required view angles around the part. In other cases, the part may attenuate x-rays too much to allow penetration in some directions or at some locations. In FIG. 1, where the x-ray source and detector are shown at 10 and 11, sections of part 12 obstruct the data acquisition scan over a portion of the angular range; even if a horizontal scan is made the x-ay path length through the part is too great to allow sufficient x-ray penetration for meaningful measurement. Incomplete data is the result, and incomplete data images generally show serious artifacts that limit the ability to properly inspect the part and detect flaws.
Previous techniques for dealing with this problem include attempts to extract relevant flaw information directly from the image with artifacts, and attempts to incorporate certain part information in an iterative fashion. The latter may include non-negativity of the pixels in an image, maximum value of the pixels in the image, and rough shape information; see U.S. Pat. No. 4,506,327--Tam and "Tomographical Imaging with LimitedAngle Input", K. C. Tam and V. Perez-Mendez, J. Opt. Soc. Am., Vol. 71 (1981), pp. 582-592. The a priori information which is available and the techniques for using it have generally been quite limited.